New supporting data to guide the use of evident toxicity in acute oral toxicity studies (OECD TG 420).

Currently there are three test guidelines (TG) for acute oral toxicity studies of substances or mixtures from the Organisation for Economic Co-operation and Development (OECD). TG 423 and TG 425 use lethality as an endpoint, while TG 420 replaces death with 'evident toxicity', defined as clear signs that exposure to a higher dose would result in death. However, the perceived subjectivity of 'evident toxicity' may be preventing wider use of TG 420. To address this, the UK National Centre for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs) and the European Partnership for Alternative Approaches to Animal Testing (EPAA) collaborated to provide recommendations on the recognition of 'evident toxicity'. Historical data from acute oral toxicity studies were analysed for clinical signs at the lower dose that could have predicted death at the higher dose. Several signs including ataxia, laboured respiration, and eyes partially closed, alone or in combination, are highly predictive. Others such as lethargy, decreased respiration, and loose faeces have lower but still appreciable positive predictive value (PPV). The data has been used to develop recommendations to promote use of TG 420 and thus reduce the suffering and numbers of animals used in acute oral toxicity studies.

Comparative pharmacokinetic studies of 14C-octamethylcyclotetrasiloxane (14C-D4) in Fischer 344 and Sprague Dawley CD rats after single and repeated inhalation exposure.

Octamethylcyclotetrasiloxane (D4) is a high production volume chemical that has been subject to thorough toxicological investigations. Animal studies with the substance were conducted with either Fischer 344 or Sprague Dawley CD rats. While the pharmacokinetic fate of D4 in Fischer rats is well understood, little information exists on Sprague Dawley CD rats, where reproductive effects have been demonstrated. The objective of this study was to explore the pharmacokinetic behavior in both rats, and to identify potential strain-specific differences. Fischer and Sprague Dawley CD rats were exposed for six hours to 700 ppm of 14C-D4 vapor either with or without preceding 14-day exposure to non-radiolabeled D4. Time-course data in blood, tissues and excreta were obtained through 168 h post-exposure and analyzed for both total radioactivity and parent D4. The data confirm that repeated exposure results in increased metabolism in both rat strains, confirming the findings of earlier studies of auto-induction of CYP2B1/2 by D4. The results also indicate that D4 is subject to strain-specific pharmacokinetic behavior, and that Fischer rats appear to metabolize D4 to a greater extent than Sprague Dawley CD rats.

Threshold of Toxicological Concern: Extending the chemical space by inclusion of a highly curated dataset for organosilicon compounds.

The Threshold of Toxicological Concern (TTC) for non-genotoxic substances, a risk assessment tool to establish safe exposure levels for chemicals with insufficient toxicological data, is based on the 5th percentile of cumulated distributions of Point of Departures in a high amount of repeat-dose, developmental and reproductive toxicity studies, grouped by Cramer Classes. The lack of organosilicon compounds in this dataset has resulted in regulatory concerns over the applicability of the TTC concept for this chemistry. We collected publicly available, scientifically robust oral repeat-dose and DART studies for 71 organosilicon substances for inclusion in the existing TTC dataset, using criteria for evaluation of studies and derivation of points of departure analogous to the Munro and COSMOS TTC publications. The resulting 5th percentile of this dataset was 13-fold higher than the 5th percentile for Cramer Class III compounds reported by Munro (which is the default for silicon-containing substances). Both the existing TTC for Cramer Class III compounds from Munro (1.5 µg/kg bw/day) and the COSMOS TTC (2.3 µg/kg bw/day), recommended by the SCCS for cosmetics-related substances, provide a conservative and sufficiently protective approach for this class of chemistry.

Challenges in working towards an internal threshold of toxicological concern (iTTC) for use in the safety assessment of cosmetics: Discussions from the Cosmetics Europe iTTC Working Group workshop.

The Threshold of Toxicological Concern (TTC) is an important risk assessment tool which establishes acceptable low-level exposure values to be applied to chemicals with limited toxicological data. One of the logical next steps in the continued evolution of TTC is to develop this concept further so that it is representative of internal exposures (TTC based on plasma concentration). An internal TTC (iTTC) would provide threshold values that could be utilized in exposure-based safety assessments. As part of a Cosmetics Europe (CosEu) research program, CosEu has initiated a project that is working towards the development of iTTCs that can be used for the human safety assessment. Knowing that the development of an iTTC is an ambitious and broad-spanning topic, CosEu organized a Working Group comprised a balance of multiple stakeholders (cosmetics and chemical industries, the EPA and JRC and academia) with relevant experience and expertise and workshop to critically evaluate the requirements to establish an iTTC. Outcomes from the workshop included an evaluation on the current state of the science for iTTC, the overall iTTC strategy, selection of chemical databases, capture and curation of chemical information, ADME and repeat dose data, expected challenges, as well as next steps and ongoing work.